In recent years, expectations for secondary batteries have been raised from a global environmental perspective. Specifically, attention has been given to secondary batteries as significant techniques for reducing the amount of CO2 emitted, because the secondary batteries are used as power sources of, for example, electric vehicles (EVs), hybrid electric vehicles (HEVs), or electric motorcycles, and are combined with natural energy electric power generation, such as solar-electric power generation or wind power generation. Among the secondary batteries, lithium ion secondary batteries have high energy density, and are expected to be reduced in size and weight. Thus, there are particularly high expectations for lithium ion secondary batteries.
Battery packs in each of which a plurality of cells are connected in parallel or in series are used to power, for example, EVs or to store power generated by solar-electric power generation, and each have a capacity or power suitable for a corresponding one of uses. In this case, the cells are efficiently arranged in a battery pack to enable an increase in volumetric energy density or gravimetric energy density of the battery pack. Thus, a process in which cells are packed is know-how that is significant for fabricators.
PATENT DOCUMENT 1 describes a process in which such cells are arranged.
In order to increase the capacity and volumetric energy density of a battery pack, lithium ion secondary batteries may be used as cells for use in the battery pack. When abnormality occurs in a lithium ion secondary battery, a material inside the cell undergoes thermal runaway, and high-temperature and high-pressure gas is generated in the cell. PATENT DOCUMENT 2 describes a sealing plate including an explosion-proof valve for safely discharging high-temperature and high-pressure gas generated inside a cell to outside the cell to address such an abnormal situation.
Furthermore, when a battery pack includes densely arranged cells, the cells are very close to one another. Thus, when one of the cells undergoes thermal runaway in an abnormal situation, the cells adjacent to the one of the cells may also be forced to start undergoing thermal runaway under the influence of high-temperature and high-pressure gas released through an explosion-proof valve. When such adjacent cells successively undergo thermal runaway, such thermal runaway may adversely affect the safety of the entire battery pack. PATENT DOCUMENT 3 describes a technique in which a duct is provided in a battery pack to safely release high-temperature and high-pressure gas released from a cell under abnormal conditions to outside the battery pack. This can reduce successive thermal runaway reactions in adjacent cells, and can ensure the safety of the battery pack.